The present invention relates to a method and an apparatus for manufacturing dried foods.
Recently, dried foods in which the original color and flavor of the raw materials remains and which gives to a person who eats the dried foods a "soft-dried and spongy" feeling pleasant to the palate, have been very popular as high-quality confectionery.
In order to provide dried foods with a "soft-dried and spongy" feeling pleasant to the palate, it is preferable to make the dried foods as porous as possible. Accordingly, it is necessary to evaporate the greater part of the moisture which is contained in the raw materials and is intended to be removed in as short time as possible after the commencement of drying process.
Heretofore, a method has been developed for drying raw materials under reduced pressure to obtain porous dried foods, such as, for example, the reduced pressure fry drying method disclosed in Japanese Patent Publication No. 36892/1986.
Two types of apparatus have also been developed for carrying out the reduced pressure fry drying method, such as (1) an apparatus having a heat source arranged outside an oil tank and being adapted to circulate by a pump the oil between the heat source and the oil tank, and (2) an apparatus having a heat source such as a steam jacket arranged within the oil tank.
As previously mentioned, i is necessary to evaporate the greater part of the moisture in the raw materials to be removed as soon as possible after the commencement of the drying process However, since both the above apparatus (1) and (2) cannot rapidly evaporate the greater part of the moisture contained in the raw materials for following reasons, neither of the apparatus (1) and (2) can produce dried foods rich in porosity.
That is, the apparatus (1) cannot supply sufficient heat in a short time since it is in fact difficult to rapidly circulate the oil by the pump when the apparatus is in a reduced pressure condition. In the apparatus (2), it is practically difficult to uniformly mix the oil in a short time under reduced pressure conditions, and it is therefore impossible to evaporate the greater part of the moisture in the raw materials to be removed in a short time after the commencement of the frying process. This is so even though the temperature of the heat source such as the steam jacket is kept at a high temperature to raise the temperature of the oil which is in contact with the steam jacket. It is also practically difficult to mix this part of oil with the other part of oil in order to maintain the temperature of the entire oil at the temperature required for carrying out the reduced pressure fry. In addition, the apparatus (2) sometimes causes unevenness of fry in the products.
There is another method for carrying out the drying process under reduced pressure conditions other than the reduced pressure fry drying method, and that is the reduced pressure microwave drying method as, for example, disclosed in Japanese Laid-Open Patent Publication No. 265046/1986. However, this method often causes glow discharge which in turn causes injurious affects on the apparatus and a partial scorching on the dried foods, which in turn causes a deterioration (e.g. oxidization) of the dried foods. In addition, unevenness of dry has occurred due to unevenness in the distribution of microwaves.
Because of the need for keeping these problems of a minimum, it has heretofore been impossible to significantly increase the output power of the microwaves. As a result, it has been impossible to evaporate the greater part of the moisture in the raw materials to be removed in a short time after the commencement of the drying process, and thus this makes it impossible to manufacture dried foods which are rich in porosity.